The present invention relates to steering wheel absolute angular position sensors.
In many applications, including motor vehicles, it can be important to know the absolute angular position of a rotating body. As but one example, when starting a motor vehicle, it may sometimes be necessary to know which steering revolution the steering wheel is in instantly at power up. Not only does this allow the driver to know which way the front wheels are directed before placing the vehicle into gear, but some computerized vehicle control systems might require knowing the steering position as well. For example, in an automated steering system, such as a steer-by-wire system, the control system must know the position of the steering wheel at all times in order to control the direction of the vehicle. Not only must these systems know the position of the steering wheel, they must know in which revolution the steering wheel is in at the time of measurement. Many of these systems of require that the steering wheel position sensor be accurate within plus or minus one degree (+/xe2x88x921xc2x0) for three hundred and sixty degrees (360xc2x0) of steering wheel rotation or within a small percentage of error for eighteen hundred degrees (1800xc2x0) of steering wheel rotation for a temperature range of minus forty degrees Celsius to one hundred and twenty five degrees Celsius (xe2x88x9240xc2x0 C. to 125xc2x0 C.).
A conventional method for determining the angular position of the steering wheel shaft includes measuring the position of the main shaft and then using a geared down position sensor to determine in which revolution the angular measurement of the main shaft was made. This method is quite simple, but it does not provide the accuracy required by present steering systems.
It happens that a more accurate method for determining the angular position of a steering wheel shaft is disclosed by U.S. Pat. No. 5,930,905 (the xe2x80x9c""905 patentxe2x80x9d), which issued in Aug. 1999 to Zabler et al. for an invention entitled xe2x80x9cMethod And Device For Angular Measurement Of A Rotatable Body.xe2x80x9d The method disclosed by the ""905 patent utilizes two gears meshed with a main shaft gear. The main shaft gear has a number of teeth xe2x80x9cmxe2x80x9d, the first additional gear has a number of teeth xe2x80x9cnxe2x80x9d that is different from the number of teeth on the main shaft gear, and the second additional gear has a number of teeth xe2x80x9cn+1xe2x80x9d. The phase difference between the two additional gears due to the additional gear tooth on the second additional gear is used to determine in which revolution the main gear shaft is in when measurement is taken. The disclosed method can provide the required accuracy, but unfortunately the device disclosed by the ""905 patent includes many parts and the method involves significant calculation in order to determine a present steering wheel revolution. As such, these geared systems can be expensive and are more likely to fail over time.
The present invention has recognized these prior art drawbacks, and has provided the below-disclosed solutions to one or more of the prior art deficiencies.
An absolute angular position sensor assembly includes an input gear, one and only one output gear meshed with the input gear, an input gear sensor to sense the angular position of the input gear, and an output gear sensor to sense the angular position of the output gear. The configuration of the output gear to the input gear is chosen so that the output gear will be out of phase with the input gear as the input gear rotates through a predetermined number of turns.
In a preferred embodiment, the input gear is attached to a rotating body, e.g., a rotating shaft. Preferably, for increased accuracy, the angular speed ratio between the input gear and the output gear is greater than or less than 1.0:1.0. Moreover, the preferred angular speed ratio is a non-integer, such as 1.05:1.0 or 1.0:1.05. In a preferred embodiment, the sensor assembly includes a microprocessor that receives output signals of the sensor and determines an absolute angular position of the input gear based thereon. Preferably, as more fully disclosed below the microprocessor determines the angular position "THgr", as being (nxc2x7360xc2x0+Y)/a.
In another aspect of the present invention, a vehicle control system includes a microprocessor and an absolute angular position sensor assembly that provides a signal to the microprocessor. In this aspect of the present invention, the signal represents an absolute angular position of an input gear.
In yet another aspect of the present invention, a method for determining the absolute angular position of a rotating body includes providing an input gear, providing an output gear, and establishing a non-integer angular speed ratio between the gears having a decimal portion. Signals are generated that represent the relative angular positions of the input and output gear and are used to calculate the absolute angular position of the input gear with relatively high accuracy. The absolute angular position of the input gear is determined as follows:
"THgr"=(nxc2x7360xc2x0+Y)/a
wherein:
"THgr"=absolute angular position of the input gear,
Y=relative angular position of the output gear,
a=gear ratio of the input gear to the output gear, and
n=number of turns of the output gear relative to the initial (zero) position.
The number of turns of the output gear is determined as follows:
n=1+int(xcex8/360xc2x0)+N*int(a), and
phase=X*axe2x88x92Y
xcex8=modulo (phase/360xc2x0) if phase greater than 0, otherwise xcex8=360xc2x0+phase
xe2x80x83Wherein:
x=relative angular position of the input gear,
N=number of turns of the input gear.
The number of turns of the input gear is related to the gear ratio, a, and determined by the phase. For instance, if a=5.2, then
xcex8=0, and N=O;
xcex8=288, and N=1;
xcex8=216, and N=2;
xcex8=144, and N=3;
xcex8=72, and N=4.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: